1. Field of the Invention
The present invention relates to a video apparatus comprising video signal supplying means and a peaking filter, which, in response to applied video signals, supplies output signals which are combined with the respective applied video signals, the peaking filter including a high-pass or band-pass filter and means for determining the peaking strength.
2. Description of the Related Art
The article xe2x80x9cVideo-Signalverarbeitungxe2x80x9d, Christian Hentschel, B. G. Teubner, Stuttgart 1998, describes a video apparatus with a peaking filter.
A peaking algorithm performs edge enhancement. This means that the steepness of an edge in a video image is measured and then the steepness may be increased by means of a peaking signal. Edge enhancement depending on the steepness of the edge, is called dynamic peaking.
The traditional implementation of peaking is to apply a 2nd-order high-pass or band-pass filter to the video signal. If the output signal of, e.g., a 2nd-order high-pass filter with an impulse response of ( . . . , 0, xe2x88x920.25, +0.5, xe2x88x920.25, 0, . . . ) is added to a video signal, this is equivalent to filtering using a filter with filter coefficients (xe2x88x920.25, +1.50, xe2x88x920.25). The step response of the 2nd-order high-pass filter results in a symmetrical peaking. Obviously, the step response of this filter has xe2x88x9225% undershoot and +25% overshoot. To avoid overflow, the amount of added peaking must sometimes be limited.
In order to determine the peaking strength, International Patent Application No. WO-A-98/09943, corresponding to U.S. Pat. No. 6,094,205, discloses adding, in parallel to the 2nd-order filter, a 1st order filter to measure the steepness of edges of video-images, followed by means for determining a gain factor, which, multiplied with the output signal of the 2nd-order filter, supplies peaking strength signals. To perform edge enhancement, these peaking strength signals are combined with the respective video signals. In order to simplify the peaking filter in WO-A-98/09943, Hentschel (see the above reference) proposes to obtain peaking strength signals by means of a look-up table (LUT) which contains peaking strength values according to a non-linear function based on the contour signals from the 2nd-order filter.
Although a 2nd-order filter with filter coefficients (xe2x88x921, +2, xe2x88x921)/2 may be used, such a high-pass filter is not the best choice. It has a maximum gain at a frequency xc2xd*fs (fs being the sample frequency), where there is guaranteed to be no more video information but only noise. Also, the perception of sharpness enhancement at such high video frequencies is not strong. It is better to use a band-pass filter, like (xe2x88x921, 0, +2, 0, xe2x88x921)/4. This filter has a maximum gain at xc2xc*fs, where there is a lot more video information. The sensitivity of the eye to that frequency is much higher. This filter has zero gain at xc2xd*fs, where there is only noise. At least that noise will not be enhanced.
It is, inter alia, an object of the invention to provide an improved peaking. To this end, according to an aspect of the invention, the video apparatus as described in the opening paragraph, comprises a first and a second 1st-order high-pass or band-pass filter in cascade configuration, and means for determining a peaking strength arranged between said first and second 1st-order filters. In a particularly simple embodiment, the first 1st-order filter and/or the second 1st-order filter is provided with filter coefficients (xe2x88x921, +1) or (xe2x88x921, 0, +1). The frequency response of such a peaking filter, which operates twice on 2 pixels, corresponds with that of a Hentschel 2nd-order filter operating on 3 pixels.
As the steepness of an edge is measured in the first 1st order filter, it may be sufficient to form the means for determining the peaking strength as a lookup-table (LUT) between the first and second 1st-order filters. In the Hentschel 2nd-order filter, the peaking strength is determined based on contour signals, while, in the filter according to the invention, the peaking strength is dependent on the slope of the edges. The 1st-order differentiator then acts to distribute the edge enhancement over the two pixels before and after the edge. Given the height of an original edge, the peaking strength, based on the first 1st-order differentiator, provides accurately how much the height will be modified.
Soft edges may be due to noise and would not be enhanced, while hard edges are already steep. Hence, the emphasis should be on enhancing only edges with medium steepness. For this reason, the look-up table (LUT) comprises peaking values which are small for small-slope steepness and for large-slope steepness, and large for medium-slope steepness.
The peaking implementation according to the invention has the following advantages: The modification of the edge height or slope is precise; there is a good trade-off between edge enhancement and noise amplification; and anti-peaking (a negative entry in the look-up table) can be used for spatial noise reduction. In these aspects, the peaking filter according to the invention is distinguished from the Hentschel peaking filter.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.